While Loop is aborting when ran

[dubbbroc]

Hello,
This kinda has to do with a function I am making for a homework assignment but it is one that I'm making. I really want to figure it out but just need a little help. I made a while loop to look for "=" signs in a string and to count how many there are. I got the while loop to do what I ask but when I run the code it says aborted at the end and it ends the entire program. Any help on why it is aborting and a direction on what I can do to stop it is greatly appreciated, I spent a lot of time trying everything I can think of.

#include <iomanip>
#include <iostream>
#include <cstring>
#include <cstdlib>
#include <string>
using namespace std;

int count_equal(string);  // function count_equal prototype

int main() {
//string qs = "firstdock&lastduckhhhhhhh";  // no equal signs for testing
string qs = "first=dock&last=duck=hhh=hh=h=h=";   // with equal signs for testing
count_equal(qs);  // function count_equal calling
cout << count_equal(qs) << endl;     // for testing
 
 return 0;

}

int count_equal(string myString) {  // function count_equal  definition
int pos = 0;
string string1;
int spot, newSpot = 1, newPos;
int count = 0;

do {
  spot = myString.find("=", pos);
  
  cout << spot << endl;  // what is returned for string.find for testing
  
  string1 = myString.substr(spot,newSpot);

  cout << string1 << endl;          // for testing
 
  if(string1 == "="){
    cout << "I can add" << endl;  // for testing
    count++;
    
  } 
	newPos = spot + 1;
    pos = newPos;
	cout << pos << endl;      // for testing
	cout << count << endl;    // for testing
} while (string1 == "=");
  return count;
}

[jlb]

If you're program is crashing you will need to post the complete program, so we can try to run the program if we so desire.

What happens if your string doesn't contain any "=" strings?

What does string.find() return when the item is not found?

[Shadowwolf]

The problem arises when your substring no longer contains any "=". myString.find will then return std::string::npos, which is a position that shouldn't exist in the string (it is usually the maximum value of size_t, which is much larger than any normal string would possibly be).

Next you pass this value to substr, which expects to get a valid index. It gets an invalid index however, and will try to create a substring from someplace not in your string, and throw an out_of_range exception. You do not handle this exception, causing the crash.

The easiest way to change this, is to make sure you get a valid index every time you execute your loop. The easiest way I can think up would be:

spot = myString.find("=",pos);
while(spot != std::string::npos)
{
    ++count;
    pos = spot + 1;
}

I would probably declare spot and pos as std::string::size_type instead of as int. This way you know that the actual positions will always fit in the variable. As you can see I also removed all irrelevant code from the actual function, including the call to substring, since it wasn't required at all. You can thrust std::string::find to really find your string, taking a substring to check it again will not make any difference.

[dubbbroc]

HI jlb,
Thanks, I edited my message so it contains entire program. I'm trying to count how many equal signs there are in a string and update the "count" integer. when I take out the equal signs the program just shows aborted. and myString.find() sends back a -1. Thanks for your help.

[jlb]

Then I take it you didn't read Shadowwolf's answer or find and read the documentation for std::string.find()?

[Shadowwolf]

The fact that you're seeing a -1 is because of signed to unsigned conversion. std::string::size_type (the actual type string.find returns) is an unsigned type. It returns the value std::string::npos when it cannot find the value, which is the maximum value it can take. When converting it to signed, it will be converted to -1 (although this isn't a guarantee, casing an unsigned int to a signed int out of the bounds of the signed int is undefined behavior, so anything could happen, from crashes to demons appearing, from signed to unsigned is defined though).

I already posted the solution around half an hour ago. Just read the post, I know it's long.

[dubbbroc]

Thanks for your help Shadowwolf! Thats makes sense, I was able to move some stuff around and got it working.

Thanks!